Silver halide emulsions containing reactive quaternary salts nucleating agents

ABSTRACT

REACTIVE N-SUBSTITUTED, CYCLOAMMONIUM QUATERNARY SALTS, WHEREIN THE N-SUBSTITUENT INCLUDES ALKYL RADICALS WHOSE TERMINAL CARBON ATOM IS ADDITIONALLY SUBSTITUTED WITH AN ACYL RADICAL SUCH AS A FORMYL RADICAL, AN ACETYL RADICAL OR A BENZOYL RADICAL ARE USEFUL NUCLEATING AGENTS IN DIRECT POSITIVE PHOTOGRAPHIC EMULSIONS.

SILVER HALIDE EMULSIONS CONTAINING RE- ACTIVE QUATERNARY SALTS NUCLEATING AGENTS Donald W. Kurtz and Judith M. Harbison, Rochester,

FY53 gzssignors to Eastman Kodak Company, Rocheser, No Drawing. Filed Oct. 30, 1970, Ser. No. 85,709

Int. Cl. G03c 1/28 U.S. Cl. 96107 R 9 Claims ABSTRACT OF THE DISCLOSURE Reactive N-substitnted, cycloammonium quaternary salts, wherein the N-substituent includes alkyl radicals whose terminal carbon atom is additionally substituted with an acyl radical such as a formyl radical, an acetyl radical or a benzoyl radical are useful nucleating agents in direct positive photographic emulsions.

The present invention relates to photography and more particularly to novel nucleating agents and directpositive photographic emulsions.

Certain photographic siliver halide emulsions are used to produce positive photographic images without requiring an intermediate processing step or a photographic negative.

Generally, these emulsions are termed direct-positive or reversal emulsions and, on exposure, they tend to form latent images predominantly inside the silver halide grains. The production of direct-positive photographic images is secured by selectively fogging the unexposed regions of a direct-positive photographic layer and developing a conventional photographic silver image corresponding to the unexposed, fogged regions. The fogging agent can be included in a fogging bath with which the imagewise exposed reversal emulsion is treated prior to image development. Alternatively, the fogging agent can be incorporated directly into the developer solution or it can be included in the reversal emulsion prior to coating a photographic layer. Heretofore, fogging agents have generally been hydrazine compounds, such as those described in US. Pat. 2,563,785 (especially at column 1, lines 18-34 and column 3, line 28 to column 5, line 23), as constituents of a surface type silver halide developer, or those described in U.S. Pat. 2,588,982 (especially at column 1, lines 21-34 and at column 4, line 1 to column 5, line 52) as included in either a surface-type silver halide developer or in the reversal emulsion itself.

When employing these techniques, and especially when the fogging agent is included in the reversal emulsion, there are certain disadvantages associated with presently known hydrazine fogging agents, or nucleating agents as they are interchangeably designated. To achieve a desirable degree of nucleation, relatively high nucleating agent concentrations, i.e., approximately 2 gm. per mole of silver as disclosed in US. Pat. 2,588,982, have been required. Such concentration levels promote nonuniform sensitometric characteristics, since undesirably large amounts of nucleating agent migrate into the developer solution. Additionally, once present in the developer, the

nucleating agent tends to promote discoloration in nonimage background areas.

It is thought that the described high nucleating agent concentrations are made necessary by the failure of nucleating compounds, i.e., hydrazines or the like, to be adsorbed to the silver halide grain. The migration of nucleating agent into the developer solution, in addition to being promoted by heavy concentration is also stimulated by the high pH, conventionally in excess of 12 and United States Patent ranging to 13 and above, which is required to maintain nucleation at an acceptable rate.

Accordingly, in view of the above and related difiiculties, it is an object of this invention to provide novel nucleating agents for photographic reversal emulsions.

It is another object of this invention to provide new, heterocyclic quaternary salt nucleating agents for photographic silver halide reversal emulsions.

Still another object of the present invention is to provide novel nucleating agents which are useful in photographic reversal emulsions at low concentrations.

Yet an additional object of this invention is to provide novel heterocyclic quaternary salt nucleating agents which are useful in direct-positive photographic elements and which do not produce substantial staining of non-image background regions.

Still another object of this invention is to provide new direct-positive silver halide emulsions wherein latent images are formed predominantly inside the silver halide grams.

These and other objects of the present invention will become increasingly apparent from a reading of the following specification and appended claims.

The objects of this invention are accomplished with direct-positive silver halide emulsions that form latent images predominantly inside the silver halide grains and which contain a heterocyclic quaternary salt nucleating agent comprising a heterocyclic nucleus containing a heterocyclic ring of from 5 to 6 atoms including a quaternary nitrogen atom, the quaternary nitrogen atom having substituted thereon a radical having the formula wherein R represents a member selected from either a hydrogen atom, a lower alkyl radical, or an aryl radical, the heterocyclic ring also having substituted thereon a lower alkyl radical (including substituted alkyl radical) which is attached to a nuclear carbon atom adjacent to the quaternary nitrogen atom in the heterocyclic ring.

Exemplary heterocyclic nuclei containing a heterocyclic ring of from 5 to 6 atoms including a quaternary nitrogen atom include such nuclei as, for example, an indole nucleus, an azole nucleus like an imidazole nucleus, oxazole nucleus, thiazole nucleus, a selenazole nucleus, or a quinoline nucleus. Representative nuclei of these types include indolenine, members of the imidazole series such as benzimidazole compounds like 5-chloro-benzimidazole and also including compounds of the naphthimidazole series; those of the thiazole series such as thiazole, 4- methylthiazole, 4-phenylthiazole, S-methylthiazole, 5- phenylthiazole, 4,5-dimethylthiazole, 4,5-diphenylthiazole, 4-(2-thienyl)thiazole, etc.; those of the benzothiazole series like benzothiazole, 4-chlorobenzothiazole, 5-chlorobenzothiazole, 6-chlorobenzothiazole, 7-chlorobenzothiazole, 4-methylbenzothiazole, S-methylbenzothiazole, 6- methylbenzothiazole, S-bromobenzothiazole, 6-bromobenzothiazole, 4-phenylbenzothiazole, S-phenylbenzothiazole, 4-metho-xybenzothiazole, S-methoxybenzothiazole, 6 methoxybenzothiazole, 5 iodobenzothiazole, 6-iodobenzothia-zole, 4-ethoxybenzothiazole, 5-ethoxybenzothiazole, tetrahydrobenzothiazole, 5,6-dimethoxybenzothiazole, S-hydroxybenzothiazole, 6-hydroxybenzothiazole, etc.; those of the naphthothiazole series such as u-naphthothiazole, 8 methoxy-tat-naphthothiazole, 7 methoxy,anaphthothiazole, naphtho [2,l-d]thiazole, etc.; those of the thionaphtheno-7,6,4,S-thiazole series like 4'-methoxythionaphtheno-7,6',4,S-thiazole, etc., those of the oxazole series such as 4-methyloxazole, S-methyloxazole, 4-phenyloxazole, 4,5-diphenyloxazole, 4-ethyloxazo-le, 4,5-dimethyloxazole, S-phenyloxazole, etc.; those of the benzoxazole series like benzoxazo-le, 5-chlorobenzoxazole, 5-

methylbenzoxazole, 5 phenylbenzoxazole, 6-methylbenzoxazole, 5,6-dimethylbenzoazole, 4,6-dimethylbenzoxazole, S-methoxybenzoxazole, S-ethoxybenzoxazole, 5- chlorobenzoxazole, 6 methoxybenzoxazole, 5 hydroxybenzoxazole, 6 hydroxybenzoxazole, etc.; those of the naphthoxazole series such as a-naphthoxazole; those of the selenazole series like 4-methylselenazole, 4-phenylselenazole, etc.; those of the benzoselenazole series like benzoselenazole, 5 chlorobenzoselenazole, 5 methoxybenzoselenazole, 5-hydroxybenzoselenazole, tetrahydrobenzoselenazole, etc.; those of the naphthoselenazole series such as a-naphthoselenazole; and those of the quinoline series such as quinoline, etc.

Included within the heterocyclic quaternary salt nucleating agents of this invention are compounds such as those having the formula wherein:

(a) Z represents the non-metallic atoms necessary to complete a heterocyclic nucleus such as those described above and containing a heterocyclic ring of from to 6 atoms including the quaternary nitrogen atom, with the additional atoms of the heterocyclic ring being selected from either carbon, nitrogen, oxygen, sulfur and selenium.

(b) R represents a member selected from either a hydrogen atom, a lower alkyl radical, or an aryl radical,

(c) R represents a member selected from either a hydrogen atom, a lower alkyl radical, an aryl radical, an aryloxy radical or an ethylene radical,

(d) n represents a positive integer having a value of 1 and when R represents an ethylene radical, a value of 2, and

(e) X represents an anion.

The heterocyclic nuclei completed by the atoms represented by Z also include those nuclei containing a heterocyclic ring of from 5 to 6 atoms including a chromophoric nitrogen atom and from 4 to 5 additional atoms of which from 3 to 4 atoms are carbon atoms and wherein the remaining additional atom is selected from either carbon, nitrogen, oxygen, sulfur or selenium atoms.

As defined herein, the term lower alkyl radical refers to alkyl radicals having from 1 to 4 carbon atoms like methyl, ethyl, propyl or butyl and preferably having 1 or 2 carbon atoms. Included within the defined alkyl radicals are substituted alkyl radicals like aralkyl radicals, e.g., benzyl phenethyl, phenoxymethyl. Also as defined herein the terms aryl radical and aryloxy radical refer to such radicals having from 6 to 10 carbon atoms in the aryl moiety, and preferably phenyl and phenoxy.

The anions represented by X include a Wide variety of acid anions including halide anions like bromide, chloride and iodide, as well as additional anions, e.g., sulfates including sulfate, hydrosulfate, and lower alkyl sulfates like methylsulfate and ethylsulfate, aromatic sulfonates such as p-toluene sulfonate and benzenesulfonate, acid anions derived from carboxylic acids like acetate, trifiuoroacetate, propionate and a wide variety of other anions including anions such as, for example, perchlorate, cyanate, thiocyanate, sulfamate, benzoate, etc.

Especially advantageous anions are halide anions like those recited above.

Particularly useful nucleating agents of the type described herein include quaternary salts such as (a) a 3-(2-formylethyl)-2-methylbenzothiazolium salt,

(b) a. 3-(2-acetylethyl)-2-benzylbenzothiazolium salt,

(c) a 3-(2-formylethyl)-2-propylbenzothiazolium salt,

(d) a 5-chloro-3-(2-formylethyl)-2-methylbenzothiazolium salt,

(e) a 3-(2-formy1ethyl)-5-methoxy-2-methylbenzothiazolium salt,

(f) a 3-(Z-acetylethyl)-2-propylbenzothiazolium salt,

(g) a 3-(2-formylethyl)-2-methylnaphtho[2,3-d]

thiazolium salt,

(h) a 3-(2-acetylethyl)-2-phenoxymethylbenzoth1azolium salt,

(i) a 2,2'-ethylenebis[3-(2-acetylethyl)benzothlazollum salt],

(1') a 2-benzyl-3-(2-propionylethyl)benzothiazolium salt,

(k) a 3-(2-acetylethyl)-2-ethylbenzothiazolium salt,

(1) a 3-(2-acetylethyl)-2-benzylbenezoselenazolium salt,

(m) a 3-(2-acetylethyl)-2-benzyl-5-phenylbenzoxazolium salt, and

(n) a 3-(2-benzoylethyl)-2-ethylbenzothiazolium salt.

The quaternary salts described herein are conveniently prepared by reacting a cyclammonium hydro salt compound with another compound containing an active double bond system such as ethylenic unsaturation between the first and second carbon atoms of an aliphatic molecule or radical.

The reaction medium can be an inert organic solvent that exhibits moderate polarity and dissolves both reagents, but which is not a solvent for the quaternary salt reaction product. Advantageous solvents include organic solvents having moderate polarity such as acetonitrile and dimethylacetamide. No reaction catalysts are generally required and the reaction temperature is conveniently variable from about 20 C. to about 30 C., although wider reaction temperatures can be employed so long as the reagents remain in solution and are not subjected to heating in excess of their decomposition temperatures. The reaction products precipitate from solution and can then be purified by such conventional means as solvent washing or sequential crystallization.

The useful acylethyl substituted quaternary salts are extremely advantageous as incorporated nucleating agents or fogging agents in silver halide reversal or direct positive emulsions utilized in photographic elements designed for reversal processing to prepare direct positive photographic images without an intermediate developed negative image. Typical of such direct positive emulsions are those that form latent images predominantly inside the silver halide grains.

The photographic emulsions generally used in reversal processes are gelatino-silver halide emulsions such as silver bromide, silver bromoiodide or silver chlorobromide emulsions. They need not contain sensitizing dyes although certain of such dyes can be added to reversal emulsions to widen spectral sensitization. Advantageous sensitizing dyes would include those described in US. Pat. 2,497,876, especially between column 2, line 15, and column 4, line 20. Internal latent image-forming emulsions are typically undigested or if digested, the digestion is carried out without the use of surface chemical sensitizers. An emulsion of this type, known as Burtons emulsion is described in Wall Photographic Emulsions, 1927, pp. 52 and 53. A further type of suitable reversal emulsion is an internal latent image emulsion described in Davey and Knott, US. Pat. 2,592,250.

An internal latent image-forming emulsion can be prepared by first forming in one or more stages silver salt grains consisting at least partly of a silver salt which is more soluble in Water than silver bromide, subsequently converting at least part of the grains to silver bromide or silver bromoiodide, ripening, preferably in the absence of ammonia, and then either washing out some of the soluble salts or washing out the whole of the soluble salts, followed the addition of soluble salts such as soluble chloride, bromide or iodide to prepare a composite silver chlorobromide, bromoiodide or chlorobromoiodide reversal emulsion. Suitable silver chlorobromide internal image emulsion also can be prepared by simultaneously adding an aqueous solution of silver nitrate and an aqueous solution containing potassium chloride and potassium bro- Grams p- Hydroxyphenylglycine 10 Sodium carbonate (crystals) 100 Water to 1 liter.

exhibits a maximum density not greater than /5 the maximum density obtained when the same emulsion is equally exposed and developed for 3 minutes at 20 C. in an internal type developer, one which develops an image inside the grains of an internal latent image emulsion, which developer type is exemplified by one having the formula:

Grams Hydroquinone 15 Monomethyl-p-aminophenol sulfate 15 Sodium sulfite (anhyd) 50 Potassium bromide 10 Sodium hydroxide 25 Sodium thiosulfate (crystals) 20 Water to 1 liter.

Preferably 'the maximum density obtained with the surface developer is not greater than ,4 the maximum density obtained when the same emulsion is developed in the internal type developer. Stated conversely, an internal latent image emulsion, when developed in an internal type developer exhibits a maximum density at least 5 and preferably at least times the maximum density obtained when the emulsion is exposed in the same way and developed in a surface" developer.

The subject quaternary salts that operate advantageously as nucleating agents or fogging agents in internal latent image-forming (reversal) emulsions are conveniently added to a silver halide reversal emulsion, such as, for example, those described in U.S. Pat. 2,566,180 or 2,592,250 in amounts designed to produce adequate fog in the form of minute surface development sites which foster the production of direct positive images upon development. The subject quaternary salt nucleating agents appear to be more tightly adsorbed to the silver halide grain surface than are previously known fogging agents, such as the hydrazine compounds of Ives, U.S. Pat. 2,5 88,- 982, since lesser amounts are required. With the utilization of a diminished amount of fogging agent, desired sensitometric characteristics are preserved since less fogging agent is released into developer solutions.

Generally, concentrations of from about 100 to about 1500 mg. of the subject fogging agents per mole of silver in the silver halide emulsion are useful with from about 200 to about 1400 mg. nucleating agent per mole of silver being preferred. These concentrations are according to conventional practice, however and with particular reversal emulsions or processing conditions more widely varying fogging agent concentrations may be desirable.

Silver halide emulsions into which the heterocyclic quaternary salt nucleating agents of this invention are advantageously incorporated typically contain other chemical addenda such as, for example, carrier vehicles, spectral sensitizing dyes, coating aids, brighteners, hardeners, developing agents and the like addenda. Desirable carrier vehicles include any of the hydrophilic, water-permeable binding materials suitable for this purpose. Suitable materials include gelatin, colloidal albumin, polyvinyl compounds, cellulose derivatives, acrylamide polymers and the like, alone or in combination and mixture. The binding agents for the emulsion layer of the photographic element can also contain dispersed polymerized vinyl compounds. Certain of such compounds are disclosed, for example, in U.S. Pats. 3,142,568 of Nottorf issued July 28, 1964; 3,193,386 of White issued July 6, 1965; 3,062,672 of Houck et al. issued Nov. 6, 1962; and 3,220,844 of Houck et al. issued Nov. 30, 1965; and include the waterinsoluble polymers and latex copolymers of alkyl acrylates and methacrylates, acrylic acid sulfoalkyl acrylates or methacrylates and the like.

Spectral sensitizing dyes useful in sensitizing the silver halide emulsions employed in this invention are described in, for example, U.S. Pats. 2,526,632 of Brooker and White issued Oct. 24, 1950 and 2,503,776 of Sprague issued Apr. 11, 1950. Spectral sensitizers which can be used are the cyanines, merocyanines, complex (trinuclear) cyanines, complex (trinuclear) merocyanines, styryls and hemicyanines.

The photographic silver halide emulsions or coatings disclosed herein can also contain non-ionic, anionic, and/ or amphoteric coating aids. Some useful coating aids include, for example, saponin, alkyl substituted aryloxyalkyleneethyl sulfonates of the type described in U.S. Pat. 2,600,831 issued June 17, 1952, maleoprimarates of the type described in U.S. Pat. 2,823,123, issued Feb. 11, 1958, taurine derivatives of the type described in U.S. Pat. 2,739,891 issued on Mar. 27, 1956, and alkyl aminopropionates of the type described in U.S. Pat. 3,133,816 issued May 19, 1964. Typical of still other good coating aids and surfactants which can be employed in the emulsion of this invention include the alkylphenoxy poly(hydroxyalkylene oxides) such as alkylphenoxy poly(glycidols) having from about 5 to about 12 glycidol units, for example, such as those disclosed in British Pat. 1,022,878 issued Mar. 16, 1966, to Olin Mathieson.

Advantageous brighteners typically include compounds containing at least one heterocyclic five or six membered ring such as derivatives of stilbene, stilbenetriazoles, triazine stilbene, cournarin, triazinylamino cournarin, oxazole, benzidene, benzimidazole, benzothiazole, benzoxazole, pyrazoline, naphthalic acid imide, etc. Exemplary brighteners include such compounds as those described in U.S. Pat. 2,933,390 and in U.S. Pat. 3,406,070.

As well as including the above-mentioned addenda, the silver halide emulsions employed in this invention can be hardened with any suitable hardener or combinations such as, e.g., formaldehyde, mucochloric acid, glutaraldehyde, maleic dialdehyde, aldehyde hardeners, aziridine hardeners, hardeners which are derivatives of dioxane, vinyl sulfones, oxypolysaccharides such as oxystarch, oxy plant gums, in organic hardeners such as chromium salts, and the like. Developing agents of the types suitable for inclusion in a silver halide emulsion are described hereinbelow.

To prepare direct positive photographic elements that can be reversal processed, an internal latent image-forming silver halide emulsion such as described hereinabove, is coated onto a typical photographic support material and dried. Advantageous support materials include conventional photographic film base materials like cellulose esters such as cellulose acetate, cellulose triacetate, cellulose acetate butyrate, etc., P'OiY-OL-OlCfiIlS like polyethylene and polypropylene, polycarbonates, polyesters such as poly(ethylene terephthalate) as well as metals such as zinc and aluminum and paper including polyethylene and polypropylene coated papers. Other support materials that are suitably used herein are known in the art.

The nucleating agents of the invention can be used in reversal color as well as black-and-white silver halide emulsions. The preparation of color reversal photographic elements is conveniently accomplished by employing cyan, magenta and yellow color-forming couplers contiguous to silver halid emulsions capable of recording light substantially complementary in color to the color of dye produced with the respective couplers on color development as described in U.S. Pats. 3,227,550 and 3,227,552.

Spectral sensitizing dyes can be used to sensitize the silver halide in the usual manner. In a multilayer element, the red-recording emulsion layer can contain a coupler such as S-(p-amylphenoxy benzene sulfonamino)-1-naphthol, the green-recording emulsion layer, a coupler like Z-cyanoacetyl-5-(p-sec. amylbenzoylamino) coumarone and the blue-recording emulsion layer, a coupler such as N-amylp-benzoylacetamino benzene sulfonate. Additional couplers are well known in the art. If desired, the color couplers can be incorporated into the developer solution, as described in Marines et al., US. Pat. 2,252,718 and Glass et al., US. Pat. 2,507,154, such an arrangement necessitating multiple exposures and color developing steps.

After an imagewise exposure, the direct positive photographic elements containing the subject nucleating agents are immersed in a conventional surface developer solution having relatively low solvent action on silver halide whereupon a position photographic image is formed. Such surface developers can advantageously contain image-enhancing compounds which increase maximum image density and lower minimum image density such as the benzotriazoles described in Stauifer, US. Pat. 2,497,917.

If color images are to be prepared, the developer solution typically contains a p-phenylenediamine color developing agent such as a 4-amino-N-dialkylaniline like those described in Mees, TheTheory of the Photographic Process, 3rd ed. (1966), pp. 294295. With a color developer, one or more colored dye images are produced depending upon the construction of the particular photographic element. In polychromatic color elements, cyan, magenta and yellow dye images are typically produced in the red sensititive, green sensitive and blue sensitive layers, respectively. Remaining silver is first converted to a soluble silver salt and removed in the usual way by treatment with a fixing agent such as sodium thiosulfate.

The following examples are included for a further understanding of the invention.

EXAMPLE 1 G. p-Methylaminophenol 2 Sodium sulfite (desiccated) 90 Hydroquinone 8 Sodium carbonate (monohydrate) 52.5 Potassium bromide 5 Water to make 1 liter.

and the developed positive image is stabilized by treatment for about 3 minutes at 20 C. with a fixing bath having the formula:

G. Sodium thiosulfate g 240 Sodium sulfite (desiccated) g 15 Acetic acid (28%) ml 48 Boric acid crystals g 7.5 Potassium alum g 15 Water to make 1 liter.

A second element (element 2) is prepared, exposed and processed in like fashion, with the exception that the subject quaternary salt fogging agent, 3-(2-formylethyl)-2- methylbenzothiazolium bromide, is added to the emulsion in a concentration of 200 mg./mole of silver. A reversal image is obtained with element 2.

Relative mu. Dmin. Gamma Control 1 1 0. 05 Element 2. 5.5 2. 5 1. 00

As can be observed, the quaternary salt of element 2 functions as an efiicient nucleating agent in reversal emulsions.

EXAMPLE 2 An internal image silver halide emulsion of the type used in layer (1) of Example 15, US. Pat. 3,227,552 and containing like addenda at similar concentrations, is coated on cellulose acetate support material. In the emulsion, 1-formyl-2-phenylhydrazide of US. 3,227,552 (which was coated at 7 mg./ft. is replaced by 3-(2-formylethyl)-2- methylbenzothiazolium bromide coated at a coverage of .14 mg./ft. The resulting red-sensitized photographic element is then exposed in an Eastman Model 1b sensitometer according to the method of Example 15 of US. 3,227,552. After exposure, the element is sequentially processed as follows:

(A) Develop for 4 min. at 37.8 C. in a developer composition having the formula:

Benzyl alcohol ml 12 Sodium sulfite g 1.5 Potassium bromide g 1.0 4-amino N ethyl 3 methyl-N-fi-methylsulfonamidoethylaniline g 7.0 Potassium carbonate (anhydrous) g 55.0 S-nitrobenzimidazole mg 5.0

Water to 1000 ml.

(pH adjusted to 11.15 with sodium hydroxide.)

(B) 37.8 C. water wash for 15 sec.

(C) Bleach-fix for 2 min. at 37.8" C. in a combination bleaching and fixing blix solution to remove de veloped silver metal and residual silver salts. The composition of the bleach-fix solution is:

Potassium ferricyanide g 35 Sodium acetate g 20 1,8-dihydroxy-3,6-dithiaoctane g 25 Acetic acid (glacial) ml 3 Water to 1 liter.

(Adjusted to pH of 5 .0 with acetic acid.)

(D) 37.8 C. water wash for 2 min.

(B) Stabilize for 2 min. at 37.8 C. in a stabilization bath having the formula:

Citric acid g 20.0 Sodium tetraborate g 14.0 Sodium hydroxide g 0.21 Formaldehyde (37% aqueous) ml 1.0

Water to 1 liter.

After processing and drying, the developed element has a positive cyan dye image with a maximum image density (D of 1.94 and a minimum image density (D of 0.25. Image quality compares favorably with those of previously reported direct positive elements although the subject formulations have significantly lowered nucleating agent concentrations. Additionally, nucleation is accomplished at a decreased developer pH compared to known hydrazine type nucleating agents (e.g. developer pH of 12.0, in Example 15 of US. 3,227,552).

EXAMPLE 3 Six pairs of direct positive photographic elements are prepared, exposed and processed as element 2 of Example 1, except that one element of each pair is processed at a pH of 12.15 which is obtained by addition of sodium hydroxide to the developer solution. The remaining element of each pair is processed at a pH of 10.5 that being the developer pH value. The quaternary salt nucleating agents, their coating coverage and the sensitometric results, including maximum image density (D minimum image density (D and image contrast (7) are summarized in the following table.

(a) Z represents the non-metallic atoms necessary to complete a heterocyclic nucleus containing a heterocyclic ring of from 5 to 6 atoms including the qua- Coverage pH-10.5 pH-12.15 (mg/mole Pair Quaternary salt silver) mux. 'Dmiu. 'y D D,,,,,,. 7

1 3-(2-ionnylethyl)-2-methylbenzothiazolium bromide) 450 0. 55 0. 10 1. 1. 22 0. 09 1. 'y 2-- 3-(2-acetylethy1)-2-benzylbenzothiazolium bromide 450 0. 94 0. 18 1. 4 3. 27 0. 20 2. 3 3... 3-(2 formylethyl)-2-propylbenzothiazolium bromide 1, 200 1. 85 0.08 4.0 2. 10 0. 07 2. 7 4..- -chloro-3-(2-fonuy1ethyl)-2-methylbenzothiazolium bromide 400 0. 88 0. 08 2. 0 1. 45 0. 1. 9 5.-. 3-(2-formylethyl)-5-methoxy2-methylbenzothiazolium bromide 400 0. 40 0. 10 0. 7 1. 30 0. 11 1. 8 6 3-(2-acetylethyl)-2-propylbenzothiazolium bromide 400 0. 38 0.08 0. 7 1. 80 0. 06 2. 1

Direct positive photographic elements are prepared, exposed and processed as in Example 3, except that 425 mg./mole Ag. of heterocyclic quaternary salt nucleating agents are employed as tabulated below:

Solveuttrom which nucleating Develagentisadded oper, Compound toemulsion pH D D i 3-(2-acetylethyl)-2-benylbenzo- Acetonitrile... 10.5 1.35 0.24

thiazolium bromide. 3-(2-acetylethyD-2-benzyl- Methanol-.- 12.1 3.28 0.21

benzozolium bromide. 3-(2-acetylethyD-2-benzyldo 10.5 0.60 0.15

benzoselenazolium bromide--- 3-(2-acetylethyD-2-benzyl- .do 12.1 2.71 0.18

benzoselenazolium bromide. 3-(acetylethyl)-2-benzyl-5- do 10.5 0.23 0.05

phenylbenzoxazolium bromide. 3-(acetylethyD-2-benzyl-5- ..-...do 12.1 2.42 0.09

phenylbenzoxazolium bromide. B-(Z-acetylethyl)-2-phenoxy- Acetonitrfle-.. 12.1 0.85 0.13

methylbenzothiazolium bromide. 3-(2-propionylethyl)-2-benzyl- Methanol/1% 12.1 2.33 0.35

benzothiazolinm bromide. image acid 3-(2-acetylethyl)-2-ethylbenzo- Methanol"--. 12.1 1. 59 0.09

thiazolium bromide. 3-(Z-acetylethyl)-2-ethylbenzo- Acetonitrile-.- 12.1 2.52 0.07

thiazolium bromide.

The above table summarizes the activity of these heterocyclic nucleating agents.

EXAMPLE 5 Two direct positive photographic elements are prepared, exposed and processed at a pH of 10.5 as in Example 3, using a like solvent of methanol/ 1% acetic acid (1:1) from which the nucleating agent is added to the emulsion at a concentration of 425 mg./mole Ag. The following quaternary salt nucleating agents are employed:

First element2,2'-ethylenebis[3-(2 acetylethyl)benzothiazolium bromide] Second element3-(2-benzoylethyl) 2 ethylbenzothiazoliurn bromide The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

We claim:

1. A direct-positive silver halide emulsion that forms latent images predominantly inside the silver halide grains, said emulsion containing a nucleating amount of a quaternary salt having the formula:

I I I I wherein ternary nitrogen atom, with the additional atoms of said heterocyclic ring being selected from the group consisting of carbon, nitrogen, oxygen, sulfur and selenium,

(b) R represents a member selected from the group consisting of a hydrogen atom, a lower alkyl radical and an aryl radical,

(c) R represents a member selected from the group consisting of a hydrogen atom, a lower alkyl radical, an aryl radical, an aryloxy radical and an ethylene radical,

(d) n represents a positive integer having a value of 1 and when R represents an ethylene radical, a value of 2, and

(e) X represents an anion.

2. A silver halide emulsion as described in claim 1 wherein, in the quaternary salt, the anion represented by X is a halide anion.

3. A silver halide emulsion as described in claim 1 wherein, in the quaternary salt (a) the alkyl radicals represented by R and R have from 1 to 2 carbon atoms,

-(b) the aryl radical represented by R and R represents a phenyl radical, and

(c) the aryloxy radical represented by R represents a phenoxy radical.

4. A direct-positive silver halide emulsion that forms latent images predominantly inside the silver halide grains, said emulsion containing a nucleating amount of a quaternary salt having the formula 6 ll X CH2CH2O-R 11 wherein:

(a) Z represents the non-metallic atoms necessary to complete a heterocyclic nucleus containing a heterocyclic ring of from 5 to 6 atoms including the quaternary nitrogen atom, with the additional atoms of said heterocyclic ring being selected from the group consisting of carbon, nitrogen, oxygen, sulfur and selenium,

(b) R represents a member selected from the group consisting of a hydrogen atom, a methyl radical, an ethyl radical and a phenyl radical,

(c) R represents a member selected from the group consisting of a hydrogen atom, a methyl radical, an ethyl radical, a phenyl radical, a phenoxy radical and an ethylene radical,

(d) n represents a positive integer having a value of 1 and, when R represents an ethylene radical, a value of 2, and

(e) X represents a halide anion.

5. A silver halide emulsion as described in claim 1 wherein the emulsion has a dye-forming coupler contiguous thereto.

6. A silver halide emulsion as described in claim 1 wherein the quaternary salt is present at a concentration of from about mg. to about 1400 mg. per mole of silver halide in said emulsion.

7. A direct-positive silver halide emulsion that forms latent images predominantly inside the silver halide grains,

said emulsion containing, as a nucleating agent, a nucleating amount of a quaternary salt selected from the group consisting of:

(a) 3 (2-formylethy1)-2-methylbenzothiazolium bromide,

(b) 3 (2 acetylethyl)-2-benzylbenzothiazolium bromide,

(c) 3 (2-formylethyl)-2-propylbenzothiaz0lium bromide,

(d) 5 chloro 3-(2-formylethyl)-2-methylbenzothiazolium bromide,

(e) 3 (2-formylethyl)-S-methoxy-Z-methylbenzothiazolium bromide,

(f) 3 (2 acetylethyl)-2-propylbenzothiazolium bromide,

(g) 3 (2 formylethyl)-2-methyl-naphtho[2,3-d1thiazolium bromide,

(h) 3 (2 acetylethyl)-2-phenoxyrnethylbenzothiazolium bromide,

(i) 2,2 ethylenebis[3-(Z-acetylethyl)benzothiazolium bromide],

(j) 2 benzyl 3-(2-propionylethyl)benzothiazolium bromide,

(k) 3 (2 acetylethyl)-2-ethylbenzothiazolium bromide,

wherein the emulsion is a gelatino-silver halide emulsion.

9. A silver halide emulsion as described in claim 2 10 wherein the silver halidecomprises silver bromoiodide.

References Cited UNITED STATES PATENTS 2,685,514 8/1954 Haist et a1. 96-454 2,887,479 5/ 1959 Heseltine 96-107 3,615,615 10/1971 Lincoln 9664 NORMAN G. TORCHIN, Primary Examiner 20 W. H. LOUIE, JR., Assistant Examiner US. Cl. X.R.

96-64 R, 94 R, 108 R 

